The long term objective of this research is to identify the basic defects in megakaryocytes and platelets in a series of 7 mouse pigment mutants which have prolonged bleeding time accompanied by platelet storage pool disease (SPD). These mice are strong candidates as animal models for such inherited human bleeding disorders as Chediak-Higashi and Hermansky-Pudlak syndromes. Identification of the altered regulatory steps in these mutants will in turn provide valuable information about steps in normal megakaryocytopoiesis and platelet granule production. The specific aims of the project are: a) To determine if inherited platelet storage pool defects in mouse mutants are due to intrinsic abnormalities in platelet precursor cells or to abnormal systemic poietic factors; b) To study the formation of dense granules, which are greatly decreased in platelets of mutant mice, in megakaryocytes of mutant and normal mice; c) To analyze molecular components of other granules such as alpha granules and lysosomes of platelets and megakaryocytes of normal and mutant mice to determine if the granular defect of mutant mice is restricted to dense granules or if more widespread abnormalities in granule biogenesis and processing occur; d) to define more specifically individual mouse mutants with regard to what is known about particular human SPD; and e) to initiate studies designed to detect the primary defective gene product in mouse models of human platelet storage pool disease. The specific methods proposed include the use of special mouse mutant stocks with SPD which are congenic and coisogenic (genetically identical) to the normal parental mouse except for the chromosomal site of the mutation. These congenic mice enable histocompatible bone marrow engraftment of mice using mutant and normal animals as reciprocal donors and hosts. The genesis of dense granules will be studied morphologically at different developmental stages of bone marrow megakaryocytes after specific incorporation of mepacrine and biochemically after using radiolabelled serotonin. Components of other megakaryocyte granules will be analyzed by immunological methods with specific antibodies to components of alpha granules and lysosomal enzymes.